294 



RADIATION BIOLOGY 



there is an immediate change in the absorption spectrum which is exactly 

 the same as that produced on irradiation (Fig. 5-3). These experiments 

 demonstrate that absorption spectrum changes caused by irradiation 

 are partly due to the action of the free radicals produced on irradiation of 

 water. 



The indirect action of ionizing radiation is also demonstrated on irradia- 

 tion of protein solutions in the presence and in the relative absence of 



240 



260 



280 



280 260 



WAVE LENGTH, m^ 

 Fig. 5-3. Effect of denaturing agents, X rays, and hydroxyl radicals on absorption 

 spectrum of protein dissolved in water. (^) Curve 1, 0.034 per cent egg albumin dis- 

 solved in water; curve 2, egg albumin dissolved in 0.07 M HCl; curve 3, egg albumin 

 dissolved in 0.07 M NaOH. {B) Curve 1, 0.034 per cent egg albumin; curve 2, 

 heated to 100°C and centrifuged soon after at 38,000 g. (C) Curve 1, egg albumin; 

 curve 2, egg albumin X irradiated with 25,000 r. (D) Curve 1, 0.07 per cent serum 

 albumin, dissolved in water; curve 2, egg albumin -\r FeSO'*. 



dissolved oxygen. At the same radiation dose the effect on the absorp- 

 tion spectrum is greater in the presence of oxygen. This is an indication 

 that hydroperoxyl radicals also contribute to these changes. 



Protein denaturation and precipitation which occur on irradiation with 

 large doses of ionizing radiation (a 0.07 per cent solution of serum albumin 

 in water becomes denatured on irradiation with 72,000 r) seem to take 

 place in two steps — the first temperature-independent, the second with a 

 large temperature coefficient. Rajewski (1930) calls the first step the 

 latent period {LZ) and believes that it can be expressed by the equation: 



